The impact of inhomogeneous subgrid clumping on cosmic reionization – II. Modelling stochasticity

Michele Bianco,Hyunbae Park,Ilian T Iliev,Kyungjin Ahn,Yi Mao,Paul R Shapiro,Sambit K Giri

doi:10.1093/mnras/stab787

Abstract

ABSTRACT Small-scale density fluctuations can significantly affect reionization, but are typically modelled quite crudely. Unresolved fluctuations in numerical simulations and analytical calculations are included using a gas clumping factor, typically assumed to be independent of the local environment. In Paper I, we presented an improved, local density-dependent model for the sub-grid gas clumping. Here, we extend this using an empirical stochastic model based on the results from high-resolution numerical simulations which fully resolve all relevant fluctuations. Our model reproduces well both the mean density-clumping relation and its scatter. We applied our stochastic model, along with the mean clumping one and the Paper I deterministic model, to create large-volume realizations of the clumping field, and used these in radiative transfer simulations of cosmic reionization. Our results show that the simplistic mean clumping model delays reionization compared to local density-dependent models, despite producing fewer recombinations overall. This is due to the very different spatial distribution of clumping, resulting in much higher photoionization rates in the latter cases. The mean clumping model produces smaller H ii regions throughout most of reionization, but those percolate faster at late times. It also causes a significant delay in the 21-cm fluctuations peak and yields lower non-Gaussianity and many fewer bright pixels in the PDF distribution. The stochastic density-dependent model shows relatively minor differences from the deterministic one, mostly concentrated around overlap, where it significantly suppresses the 21-cm fluctuations, and at the bright tail of the 21-cm PDFs, where it produces noticeably more bright pixels.

Highlights

The Epoch of Reionization (EoR) is an important period in the history of the Universe, which encompasses the cre-IN ation of the first stars and galaxies that subsequently influenced the formation and evolution of latter-day structures
D radiative transfer cosmic reionization simulation code, in orEder to present the impact of various modeling approaches for gas clumping on reionization observables
Due to computational limitations which limit the dynamic range, D uniformly high resolution cannot be achieved in such a volE ume

Summary

Introduction

The Epoch of Reionization (EoR) is an important period in the history of the Universe, which encompasses the cre-IN ation of the first stars and galaxies that subsequently influenced the formation and evolution of latter-day structures. The key goal of reionization simulations is to provide numerical framework for constraining EoR observables, for example the detection of the 21cm hyperfine transition of neutral hydrogen fluctuations (Bowman & Rogers 2010; Paciga et al 2013; Yatawatta et al 2013; Parsons et al 2014; Jelic et al 2014; Jacobs et al 2015; Dillon et al 2015; Robertson et al 2015; Ali et al 2015; Pober et al 2015; Patil et al 2017; Mertens et al 2020; Ghara et al 2020) and Lyman-α damping wings (Davies et al 2018; Greig et al 2019)

Methods

Results

Conclusion